Ultra Wide Band (UWB) radio technology may differ from conventional narrow band radio and spread-spectrum technologies in that the bandwidth of the signal is greater, for example, the bandwidth being at least 500 MHz wide or the bandwidth of the signal at −10 dB being 20% greater than the center frequency bandwidth. Further, in UWB pulsed technology, instead of transmitting a continuous carrier wave modulated with information or with information combined with a spreading code, which determines the bandwidth of the signal, a UWB pulsed signal comprises a series of narrow pulses, typically, less than or around 2 ns in duration. These short time-domain pulses, when transformed into the frequency domain, result in the ultra-wide band spectrum of UWB radio (also called UWB impulse radio).
In UWB, the information carried by the pulsed signal can be coded, for example, by using a pulse position modulation. In other words, the information coding is performed by modifying the emission time instant of the individual pulses. More precisely, each pulse is transmitted in a window having a predetermined duration, called “pulse repetition period” (PRP). The transmitted pulse is thus before or behind a reference emission position, which permits encoding a “zero” (0) or a “one” (1). It is also possible to encode more than two values by using more than two positions shifted with respect to the reference position, and to superimpose a BPSK modulation on this position modulation.
UWB impulse radio has been approved by the IEEE 802.15.4a standardization body as a radio technology suitable to enable low-cost and low-power devices for LDR applications within ad hoc sensor networks. Besides interference mitigation and network co-existence enhancements, one of the key drivers for the development of an alternative LDR physical layer over the existing ZigBee/IEEE-802.15.4 approach is to provide the protocol and radio support for accurate ranging and localization applications. Compared to the narrowband carrier modulation system, the impulse-based UWB-LDR occupies a large bandwidth, i.e. roughly 500 MHz, which directly translates into high multipath resolvability. Thus, it may be well suited for accurate ranging by estimating the Time-of-Flight (ToF) of the transmitted signal.
Since ad hoc sensor networks are typically comprised of many asynchronous devices with no common reference clock, a time-multiplexed half-duplex ranging protocol between two distant nodes has been provided in the IEEE 802.15.4a standard to provide accurate ranging through ToF measurements.
Since the ToF is only a fraction of a microsecond, the error that may be caused by the quartz crystal inaccuracies—which is expected to be as large as ±40 ppm for low-cost devices—is the relative crystal drift. The standard defines two ways to manage this relative crystal (or clock) drift, referred to as implicit and explicit ranging modes. The explicit ranging mode uses the receiving node to estimate the relative drift and to report it back to the transmission node.
At present, the IEEE 802.15.4a standard appears to give no indication on how to implement such a relative drift estimation. However, every standard-compliant apparatus may have to implement a crystal (or clock) drift estimation. Whereas the clock drift is an important question for ranging applications in UWB-LDR systems, such a clock drift is generally more a problem in UWB systems because it can lead to a synchronization loss during communication.
In narrowband systems, non-coherent delay lock loop (DLL) designs have been used extensively to ensure accurate synchronization based on Early-Late Gate Synchronizer (ELGS) circuits. ELGS circuits exploit the symmetry property of the correlation function or the matched filtering between the received signal waveform and its replica. The ELGS monitors two equally spaced sampling points, where one sampling instant is running late and the other one is running early with respect to the eligible “on time” sampling point, thereby compensating for any timing misalignments in a continuous fashion within a feedback loop.
Due to the very large bandwidth of UWB signals, the main lobe of the cross-correlation function is relatively narrow and may decay rapidly, which may result in high sampling frequency constraints to recreate the waveform template in order to exploit the ELGS symmetry property. Further, signal waveform replicas may be difficult to detect for UWB signals in realistic unknown multipath channels. Moreover, a high sampling frequency (typically 4 to 8 times the bandwidth), being necessary for the received signal to extract the early and late samples in ELGS circuits, leads to a high power consumption. And, such high power consumption is generally a disadvantage, more particularly, for low cost and low power devices intended to be used in UWB-LDR applications.
On the other hand, lowering the sampling frequency, for example, sampling around Nyquist rate, which is used to limit the power consumption, limits the accuracy of the digital signal representation. Thus, using an ELGS for estimating the clock drift may be difficult to implement in the context of UWB pulsed signals, especially for low power sensor devices.